CHAPTER X. 

 INTERNAL FACTORS. 



§ 153. We saw at the outset (§§ 10 — 16)^ that organic 

 matter is built up of molecules so extremely unstable, that 

 the slightest variation in their conditions destroys their 

 equilibrium ; and causes them either to assume altered 

 structures or to decompose. But a substance which is beyond 

 all others changeable by the actions and reactions of the 

 forces liberated from instant to instant within its own 

 mass^ must be a substance that is beyond all others change- 

 able by the forces acting on it from without. If their 

 composition fits organic aggregates for undergoing with 

 special facility and rapidity those re-distributions of matter 

 and motion whence result individual organization and life : 

 then their composition must make them similarly apt to 

 undergo those permanent re-distributions of matter and mo- 

 tion which are expressed by changes of structure, in corre- 

 spondence with permanent re-distributions of matter and 

 motion in their environments. 



Already in Fii^st Principles, when considering the phe- 

 nomena of Evohition in general, the leading characters and 

 causes of those changes which constitute organic evolution, 

 were briefly traced. Under each of the derivative laws of 

 force to which the passage from an incoherent, indefinite 

 homogeneity to a coherent, definite heterogeneity, conforms, 

 were given illustrations drawn from the metamorphoses of 



